A conventional organic light-emitting diode (OLED) is formed of several stacked nano-size layers including an anode, a hole transport layer (HTL), a luminescent layer, an electron transport layer (ETL) and a cathode in spatial order. When a voltage is applied to an OLED, a current of holes flows from the anode to the highest occupied molecular orbitals (HOMO) of the HTL, thus generating positive polarons, and at the same time a current of electrons flows from the cathode to the lowest unoccupied molecular orbitals (LUMO) of the ETL, thus generating negative polarons. The positive polarons and the negative polarons recombine in the luminescent layer, thereby generating singlet excitons and triplet excitons. Thereafter, the singlet excitons return to the ground state, upon which light is emitted.
In the evolution of OLEDs, extending the lifetime of blue OLEDs has been a key issue, the reason being that the energy of blue photons is relatively high, which results in the property of rapid degradation. To be specific, active exciton-polaron annihilation exists in blue OLEDs since excitons possess long lifespan and thus tend to react with excitons and form high energy polarons that break molecular bonds in the luminescent material and consequently reduce the lifetime of blue OLEDs.
In addition, light is emitted when singlet excitons return to the ground state, while triplet excitons in fluorescent materials cannot return to the ground state with photon emission. That is to say, a considerable amount of energy is wasted taking into consideration of the fact that triplet excitons account for 75 percent of excited excitons as a result of the recombination between the positive polarons and the negative polarons.
Therefore, in light of the above-mentioned drawbacks, OLEDs of the prior art still have room for improvement.